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A NEW CLASS OF IN-BAND MULTITONE TEST SIGNALS
Jon M. Risch, Peavey Electronics Corp., Meridian, MS. USA            Copyright 1998

Presented at the 105th Convention of the Audio Engineering Society
1998 September 26-29th, San Francisco, California as preprint #4803

CONT'D

3.0  Experimental Results

Experimental multitone test signals were generated using a PC based waveform generator operating in the digital domain, and transferring the final product to CD-R for playback and use.

It was found that some CD players were unable to handle output to full 0 dBFS, with the result a much higher level of distortion.  It was as if some component was either clipping or compressing the signal, generating harmonics, intermodulation and crossmodulation products all throughout the spectrum.  Accordingly, it was found that reducing the level of the digitally generated recorded signal by 2 dB was enough for most players to avoid this problem.  In order to avoid any vestige of amplitude related compression or clipping, the signals were generated and recorded with peak signal levels below -3 dB.  Some players still had a problem with these peak levels, but most did not.  The author suspect’s the digital filtering within the CD player may be the cause.

    3.1  Revisions
   
The original Phi12 stimulus had some distortion product cover up, due to some of the intermediate tones intermodulating with each other, and generating the same frequency.  Once this was discovered, a revised version that used different multipliers for each of the 6 base tones was devised.  While this did include some arbitrary multiplier choices, and could conceivably be improved, it was found useful as a more complex stimulus than the 6 tone version.

Accordingly, from the base frequency of 100 Hz, a multiplier of 1.22 was used, resulting in 122 Hz.  From 261.8 Hz, a multiplier of 1.33 was used.  resulting in 348.2 Hz, from 685.4 Hz, a multiplier of 1.44 resulting 987.0 Hz, from 1794 Hz a multiplier of 1.6 resulting in 2870.4 Hz, from 4697.9 Hz, a multiplier of 1.44 resulting in 6765.0 Hz, from 12299 Hz, a multiplier of 1.33, resulting in 16358 Hz.  None of these has obvious intermodulation or crossmodulation products that occur at exactly the same frequency.  Most distortion products are far enough apart for a spectrum analyzer to differentiate between the different products.  Figure 3 is the FFT spectrum of this signal, from a CD player through a mixer.

4.0  Further Test Signal Refinements

Other changes and refinements were considered to reduce or alleviate some of the limitations of the signal.

    4.1  Possible Crest Factor Reduction

Proper phasing of the various signal tones can provide some relief, but a DUT that had significant amplitude changes, roll-off or phase shifts would destroy the carefully orchestrated phase relationships of the original test signal.  It is recommended that full allowance be made for the worst case crest factor of the multiple tones.  Several references regarding such phase manipulation are provided. (4)

    4.2  Split Octave Bands - High Low

After working experimentally with some of the variations on the signals, it was determined that more tones or a higher density of tone spacing might be desirable to highlight intermodulation and cross-modulation distortions, and provide more distortion products across the audio band.  It was also determined that limiting the total number of individual tones would help preserve dynamic range.

To raise the tone interval density without increasing the number of tones to a high number, isolated octave bands of tones was considered.  A suitably non-overlaying multiplier was desired, so a multiplier of one plus one tenth the original full audio band multipliers were investigated.  Scatter diagrams were used to evaluate the suitability of various octave band combinations with a reasonable total number of tones, that would provide a wide range of harmonic, intermodulation, and crossmodulation products.  One of the concerns was to limit the spread of each group or band of signals to an octave or less, so that the second harmonics of the band would not be covered up by the primary tones.

A pair of split octave bands was found to provide an efficient and useful multitone test signal without an excessive number of individual tones.  An octave starting at 100 Hz, in conjunction with an octave starting at 5 kHz, with both containing five tones, and with spacing multipliers of 1.1618.  This results in frequencies of 100, 116.18, 134.98, 156.80 and 182.19 Hz for the low band, and of 5000, 5809, 6748.9, 7840.9, and 9109.6 Hz.  This test signal proved particularly efficacious at ferreting out intermodulation and crossmodulation in two-way loudspeakers.  Figure 4 depicts the FFT spectrum of this signal, from a CD player through a mixer.

Note that this is similar in spirit to the third signal proposed by Sokolich and Jensen, but not identical.  Much larger areas are left open for distortion products to manifest and be readily observed.

Other split band combinations were investigated, such as: 

100 -182 Hz plus 10 - 18.2 kHz, with frequencies of 100, 116.18, 134.98, 156.80 and 182.19 Hz for the low band, and 10000, 11618, 13498, 15682, and 18219 Hz for the high band.  Figure 5.

100 -182 Hz plus 1.11 - 2.02 kHz, with frequencies of  100, 116.18, 134.98, 156.80 and 182.19 Hz for the low band, and 1109.0, 1288.4, 1496.9, 1739.1, 2020.5 Hz for the high band.  Figure 6.

261 - 477 Hz plus 1.11 - 2.02 kHz,  with frequencies of 261.80, 304.16, 353.38, 410.56, and 476.99 Hz for the low band, and 1109.0, 1288.4, 1496.9, 1739.1, 2020.5 Hz for the high band.  Figure 7.

261 - 477 Hz plus 10 - 18.2 kHz, with frequencies of 261.80, 304.16, 353.38, 410.56, and 476.99 Hz for the low band, and  10000, 11618, 13498, 15682, and 18219 Hz for the high band.  Figure 8.

685 - 1249 Hz plus 1.79 - 3.27 kHz, with frequencies of 685.41, 796.31, 925.16, 1074.9, and 1248.8 Hz for the low band, and 1794.4, 2084.8, 2422.1, 2814.0, and 3269.3 Hz for the high band.  Figure 9.

    4.3  Revised Split Octave Band Signal

It proves all too easy to fall into the same trap as before.  In selecting even starting frequencies, such as 100 Hz for the low band, and 10 kHz for the high band, some of the harmonics, intermodulation  and crossmodulation products line up again, covering up the source of the tones responsible for the distortion.

After trying a different start frequency that was not an even multiple of the other start frequency for the high band or the low band, it was realized that by using the same multiplier for each band, the top band of frequencies would divide into the bottom band and have the same crossmodulation product.  This leads to the need for a different multiplier for the separate bands.  After checking for this, a slightly different multiplier for one of the bands was used, 1.2618 vs. 1.1618. 

Use of the new 1.2618 multiplier results in the high frequency band extending for a bit more than an octave, but the author considers this to be completely arbitrary and a minor issue.  If a band of closely spaced multitone frequencies extends past an octave, some potential exists for cover-up of harmonics.  For the higher frequency bands where this was done, the resolution is sufficient to allow determination of the 2nd harmonics without difficulty.  This is why the low frequency band uses the originally derived multiplier.

At this point, a spread sheet was developed for use in evaluating the resultant distortion products that would occur with various primary test signal tones in a spectral type test signal.

Figure 10 is a copy of the spread sheet for the Phi6 spectral signal, Figure 11 is a scatter diagram of the primary tones for the Phi6 spectral, harmonics (up to the tenth), and intermodulation and crossmodulation products.  Figure 12 is the spread sheet for the Phi12 spectral signal, and Figure 13 the scatter diagram.  Figure 14 is the spread sheet for the Phi12r (revised), and Figure 15 the scatter diagram.  Figure 16 is the spread sheet for the first split band spectral, the one with bands at 100 -182 Hz plus 5 - 9.1 kHz.

Experimental results for the first set of split band spectral test signals reveal that the pairings with the low band at 261 to 477 Hz did not show any additional distortion products than the ones with the low band from 100 to 182 Hz.  They also do not show low frequency second harmonics as readily.  Also, the pairings that had the high band in the last octave do not show any high frequency second harmonics, or intermodulation or crossmodulation products on the high side before the audio band was exceeded.  In order to address this,   revised split band signals were reduced to a set of low and high band signals, and a set of low and middle band signals.

The revised split band spectral’s now use frequencies at:  100 -182  Hz plus 4.7 - 11.9 kHz, with frequencies of 100, 116.18, 134.98, 156.80 and 182.19 Hz for the low band, and 4697.9, 5927.8, 7479.7, 9437.9 and 11909 Hz for the high band for the low/high band pairing, and called a Phi Low-High split band spectral.  Shown in Figure 17.

The low/middle revised split band now uses frequencies at:  100, 116.18, 134.98, 156.80 and 182.19 Hz for the low band, and 986.99, 1245.4, 1571.4, 1982.8 and 2502.0 Hz, and I call it a Phi Low-Middle split band spectral.  Figure 18.

    4.4  Phi Tri-Band Spectral Signal

Further work with these split band spectral signals resulted in the development of a split band signal with components at low, middle AND higher frequencies, which left two large open areas for distortion products to show up.  The multipliers have to be different for each band.   The total number of tones needs to be kept down, so three clusters of four frequencies was used, for a total of 12 separate tones.  Multipliers of 1.1618, 1.2618 and 1.12618 were used for the three bands respectively.  The frequencies that result are:  100, 116.18, 134.98, and 156.80 Hz for the low frequency band, 986.99, 1245.4, 1571.4, and 1982.8 Hz for the middle band, and 6764.9, 7618.5, 8579.8 and 9662.5 Hz for the high frequency band.  I refer to this one as a Phi tri-band spectral, shown in Figure 19.

The frequency ranges were chosen based on earlier work with split band multitone’s, and what the author felt would be good ranges for excitation frequencies.   Each band has an empty space of approximately two octaves in-between,  the low band two octaves below, and the high band an octave above (within the audio band).  The start frequency of each band is based on the revised Phi12 (Phi12r) spectral contamination test signal.  The underlying premise is that none of the frequencies within each band, including the start frequency, would encourage distortion products to be covered up by other products, or by the primary tones.

Cont'd

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